Exhaust after-treatment systems receive and treat exhaust gas generated from an internal combustion engine. Typical exhaust after-treatment systems include various components configured to reduce the level of harmful exhaust emissions present in the exhaust gas. For example, some exhaust after-treatment systems for diesel powered internal combustion engines include various components, such as a diesel oxidation catalyst (DOC), a particulate matter filter or diesel particulate filter (DPF), and an SCR catalyst. In some exhaust after-treatment systems, exhaust gas first passes through the DOC, then passes through the DPF, and subsequently passes through the SCR catalyst.
Each of the DOC, DPF, and SCR catalyst components is configured to perform a particular exhaust emissions treatment operation on the exhaust gas passing through the components. Generally, the DOC reduces the amount of carbon monoxide and hydrocarbons present in the exhaust gas via oxidation techniques. The DPF filters harmful diesel particulate matter and soot present in the exhaust gas. Finally, the SCR catalyst reduces the amount of nitrogen oxides (NOx) present in the exhaust gas.
SCR catalyst systems utilize a diesel exhaust fluid (DEF) or reductant to reduce NOx in exhaust gas. Typical SCR systems include a DEF delivery system that includes a DEF source, pump, and delivery mechanism. The DEF source can be a container or tank storing a DEF, such as, for example, urea solution or ammonium formate solution. The pump supplies DEF from the source to the delivery mechanism via a DEF line. The delivery mechanism, which typically is a DEF injector, delivers the DEF into an exhaust gas stream upstream of an SCR catalyst. In automotive applications, the DEF typically is urea, which decomposes to produce ammonia. After reduction, the ammonia reacts with NOx in the presence of the SCR catalyst to reduce NOx to less harmful emissions, such as N2 and H2O.
For proper operation, the temperature of the DEF stored in the DEF storage tank and pumped through the DEF line between the tank and delivery mechanism must be maintained above the freezing point of the DEF and below a maximum temperature of the DEF. Emissions regulations require SCR systems to provide a temperature control system for heating the DEF when operating at low ambient temperatures. Some conventional DEF temperature control systems use engine coolant to heat DEF stored in the storage tank and line. Other conventional DEF temperature control systems employ electrical heaters instead of coolant to heat DEF in the storage tank and line.
Generally, for proper operation, conventional DEF temperature control systems require an ambient air temperature sensor as the primary input. The ambient air temperature, as detected by an ambient air sensor, is required as a reference point from which a proper heating temperature, i.e., an upper heating limit, is established. Unfortunately, however, ambient air temperature sensors may have various tolerances depending on the sensor mounting location and/or vehicle operating conditions. For example, ambient air temperature sensors may be mounted in different locations for different engine platforms. Also, different engine platforms may use ambient air temperature sensors in different ways to detect ambient air temperature.
Sensor tolerances may affect the accuracy of the ambient air temperature readings. For example, an ambient air temperature sensor may be mounted in a location that tends to cause either positive or negative errors in the ambient air temperature sensor readings. Further, an ambient air temperature sensor may be defective or become inaccurate over time based on limits and mounting locations of the sensor, which may lead to an increased risk of refreezing or overheating the DEF.